There are a couple of thrid-party tools that can do this for you.
See:
Silicon Prairie's Paragraph & Character Tools: http://www.siliconprairiesoftware.com/Products.html
Cudspan's Template Mapper: http://www.cudspan.net/plugins/executioner.php?do=displayPage&field_ta b_num=2
Alternatively, you can do this manually, one tag at a time, using the Designers. Place the cursor in a paragraph (or character tag). You'll see the current name in the Status Bar. Then change the name in the Paragraph Tag line, e.g. from Normal to Body, and click on the Update All button. FM will ask you if you want to "Rename all Normal tags to Body?" and click on the OK button.